This invention relates to an improved capacitance humidity sensing element for use in humidity measuring and control systems.
Humidity can be measured by a number of techniques which are based upon the reversible water absorption characteristics of polymeric materials. The absorption of water causes a number of physical changes in the polymer. These physical changes can be transduced into electrical signals which are related to the water concentration in the polymer and which in turn are related to the relative humidity in the air surrounding the polymer. Two of the most common physical changes are the change in resistivity and the change in dielectric constant which can be respectively translated into a resistance change and a capacitance change. Arrangements utilizing the resistance change may, for example, be constructed in accordance with U.S. Pat. No. 3,559,456 issued on Feb. 2, 1971, to F. Lomker et al. It has been found, however, that elements utilized as resistive components suffer from the disadvantage that there is an inherent dissipation effect caused by the dissipation of heat due to the current flow in the elements necessary to make a resistance measurement. The result is erroneous readings.
Elements constructed to approximate a pure capacitance avoid the disadvantages of the resistive elements. However, it is important in the construction of capacitive elements to avoid the problems which can arise with certain constructions for such elements. Exemplary of the capacitive type element is that shown in U.S. Pat. No. 3,802,268 issued to Paul E. Thoma on Apr. 9, 1974. In that patent a sheet of cellulose acetate butyrate is sandwiched between two planar electrodes, one of which is porous to allow water molecules to equilibrate with the bulk of the film. Among the problems which are encountered with this type of construction is the slow response due to the thickness required to support the structure. There is also a difficulty in fabricating a conductive yet porous electrode. In addition, there is also inaccuracy incurred at high relative humidity values in that the high water content causes problems due to excessive stress and the resulting mechanical shifts in the components of the element.
By making the component parts of the element thin, the above mentioned problems can be avoided and the capacitance type element can provide a fast, precise measurement of the relative humidity content of air over an extreme range of humidity as well as over an extreme range of temperature and pressure and other environmental variables.
Humidity sensing elements of the capacitance sensing type usually include a moisture insensitive, non-conducting structure with appropriate electrode elements mounted or deposited on the structure along with a layer or coating of dielectric, highly moisture sensitive material overlaying the electrodes and positioned so as to be capable of absorbing water from the surrounding atmosphere and reaching equilibrium in a short period of time.
This invention discloses a capacitive sensing element in which all of the difficulties of the above mentioned element of U.S. Pat. No. 3,802,268 are avoided. This improved structure utilizes a planar interdigitated electrode structure to form the capacitor. The electrode structure is advantageously made of thin film metal deposited on a suitable non-conducting substrate and patterned to form two sets of interdigitated fingers. Each set of the interdigitated fingers is connected in parallel to a separate bus and a contact. The capacitance between the two sets of fingers is, of course, determined by the spacing between the fingers, the width and length of the fingers, the number of fingers, and the dielectric constant of the material applied over the fingers to coat the surface of the element. The humidity sensitivity of course arises from the humidity-related dielectric constant changes which occur in the coating over the fingers, for all other parameters remain constant. In the present invention, a polymer such as cellulose acetate butyrate is utilized to provide the humidity-related dielectric. This polymer coating must, of course, be made as thin as possible in order to have a short response time to changes in the relative humidity of the surrounding atmosphere. It has been found that one of the difficulties which arises with such a structure is the contamination of the outer surface of the polymer which can lead to a condensation on the surface at high humidities and which therefore leads to very non-linear and non-repeatable outputs from the elements. It is therefore an object of this invention to avoid such difficulties with contamination while maintaining a minimum response time for the elements.